1. Field of the Invention
This invention relates to spool valves serving to provide switching operation on pressurized air or like pressure medium and, more particularly, to a spool valve arrangement suited for forming spool valve components from synthetic resin.
2. Description of the Prior Art
A well-known spool valve which has its structural parts formed from synthetic resin, has an arrangement having an injection-molded plastic casing, as disclosed in, for instance, Japanese Patent Laid-Open Publication No. 11-223267. Also, various valve structures including a spool valve constituted by resin parts are disclosed in, for instance, Japanese Patent Laid-Open Publications Nos.11-201304, 9-229214, 8-233132, 2000-178447 and 2001-41331.
The spool valve which has its structural parts formed from synthetic resin, is desired in that compared to the spool valve made of metal, it is possible, owing to weight reduction of the spool, to reduce the pilot pressure using pressurized air or the like for reciprocation, as well as obtaining smooth and quick spool valve switching operation and saving the drive energy. In the mean time, the spool valve mainly comprises a spool, a cylinder which reciprocally accommodates the spool in its spool bore and a housing having a cylinder bore with the cylinder insertedly mounted therein. A spool valve in which the spool and the cylinder are formed from metal, is desired so far as the accuracy of the clearance between the spool and the cylinder can be held within several microns. However, a problem is posed in that foreign particles may enter the clearance, that is, so called foreign particle catch may occur, giving rise to the locking of the spool. By forming the spool and the cylinder from synthetic resin, the above phenomenon of foreign particle catch can be reduced because the clearance is increased to about 40 microns owing to the accuracy concerning the synthetic resin. In a further aspect, a spool valve in which the spool and the cylinder for guiding the spool are formed from metal, usually uses O-rings made of rubber or the like for sealing frictional parts and also use grease coated for improving the sliding. Grease, however, poses a problem in that it may enter discharge air or like pressure medium. For example, where an apparatus to be driven by switching operation of the spool valve is a bellows pump for controlling semiconductor processing liquid, pressure medium contaminated by grease may have adverse effects as oil mist on the purity of the semiconductor processing liquid. Particularly, with an apparatus constituted by a clean room, a serious problem will occur because of contamination of the inside of the apparatus by oil mist introduced into discharge air.
A spool valve which uses ceramics for its material is desired in that it has excellent durability and may have high accuracy. However, a problem is posed in that the material cost is high. Also, this material is weak to shocks, thus giving rise to problems in transport and maintenance. Also, the ceramics are heavy compared to plastics, and a spool valve made of ceramics can difficultly cope with fast operation. Furthermore, since the ceramics are hard, end places to be in contact with the spool at the operation stroke ends are worn greatly, thus giving rise to problems of great stroke end position errors and unstable operation.
A spool valve which uses synthetic resin for its constituent parts, that is, a full resin spool valve, can solve the above problems. This spool valve, however, has the following problems. The cylinder inserted into the cylinder bore of the housing is usually secured by pressure fitting. The pressure fitting of the synthetic resin cylinder, however, results in large deformation, i.e., large deformation of the inner diameter of the cylinder and the inner diameter of the spool valve, giving rise to the possibility of interference with smooth movement of the reciprocated spool in the spool bore. In case of forming two constituent parts from synthetic resins having different linear expansion coefficients, such problem as deformation of the mutually fitted parts due to influence by temperature may arise. To avid this, it may be thought to fit the cylinder by clearance fit in the cylinder bore. In this case, however, a special stopper for preventing rotation of the cylinder relative to the housing is necessary. A further problem resides in that pressure medium leaks greatly via pressure medium ports provided in the housing and the cylinder into the clearance between the housing and the cylinder. This leads to erroneous operation of the spool valve, making it impossible to obtain stable operation.
The invention was made in view of the above various problems inherent in the prior art, and it has an object of providing a spool valve arrangement, which, although its constituent parts are made of synthetic resin, can solve the above problems, permits fast spool operation, can maintain stable operation at low cost, is richly durable and can be readily assembled.